In a semiconductor manufacturing apparatus, liquid crystal panel manufacturing apparatus, or the like, a substrate conveying robot is used to convey a substrate, such as a semiconductor wafer or a glass substrate, to a desired position precisely while keeping the substrate clean. The substrate conveying robot includes an end effector, which is a suction hand, for example. By means of the suction hand, the substrate conveying robot retrieves a substrate from a cassette in which substrates are stored in a multi-staged manner, and then conveys the substrate while holding the substrate on the hand.
Conventionally, detection of the presence or absence of a substrate in a cassette has been performed by using an optical sensor provided on the distal end of a hand of a robot (see Patent Literature 1, for example). In recent years, a substrate conveying robot capable of quickly detecting the presence or absence of a substrate in a cassette has been disclosed (see Patent Literature 2, for example). The substrate conveying robot includes a hand provided with two different types of optical sensors. A first sensor is installed such that its optical axis is horizontal, and a second sensor is installed such that its optical axis is inclined by a certain angle relative to the horizontal direction.
In the case of a conventional suction end effector, the end effector may not be able to sufficiently suck and hold a substrate if the substrate is bent. If the substrate is conveyed in such a state, there is a risk of the substrate falling from the end effector. Therefore, it is necessary to check whether the sucking and holding of the substrate are sufficient and whether the substrate is stably placed on the end effector.
In this respect, for example, based on information from a pressure switch provided on the hand, the conventional substrate conveying robot determines whether or not a substrate on the hand is bent, thereby detecting whether or not the substrate is properly sucked and held. Patent Literature 3 discloses a conveying robot configured to detect, by means of a transmissive optical sensor provided on a hand, whether or not a substrate has been properly held.